The present invention relates to an in-cylinder direct injection spark-ignition internal combustion engine, and specifically to a so-called DI fuel injector of a direct injection spark-ignition engine that uses at least two combustion modes, namely a homogeneous charge combustion mode (an early injection combustion mode) where fuel-injection early in the intake stroke produces a homogeneous air-fuel mixture, and a stratified charge combustion mode (a late injection combustion mode) where late fuel-injection delays the event until the end of the compression stroke to produce a stratified air-fuel mixture.
In recent years, there have been proposed and developed various swirl DI fuel injectors suited to direct-injection (DI) spark-ignition gasoline engines. The swirl DI fuel injector often uses a swirl flow component generating means, such as a swirler located upstream of a conically or semi-spherically ended needle valve or a spiral guide groove formed on the outer periphery of the needle valve, in order to give rotational momentum to fuel. One such swirl DI fuel injector has been disclosed in Japanese Patent Provisional Publication No. 8-42427 (hereinafter is referred to as xe2x80x9cJP8-42427xe2x80x9d) and assigned to the assignee of the present invention. In comparison with a non-swirl-flow generating-means equipped DI fuel injector, the swirl DI fuel injector can inject fuel spray of a larger spray angle, thereby enabling a wide range of fuel injection, while effectively atomizing the fuel. Such a swirl DI fuel injector has a peculiar fuel spray characteristic that the shape of fuel spray is a substantially hollow cone and a spray angle is dependent upon an ambient pressure (i.e., a pressure level measured at the outlet side of a fuel injector orifice nozzle hole) as well as a fuel pressure. Concretely, the spray angle of fuel injected tends to become smaller, as the ambient pressure rises. The previously noted peculiar fuel spray characteristic of the swirl DI fuel injector is suitable to a DI gasoline engine equipped with an open combustion chamber type piston in which a piston bowl cavity combustion chamber is recessed in a piston crown and a stratified combustion mode is realized by means of the piston bowl cavity. In such a piston-bowl-cavity equipped DI engine, at partial loads, the engine operates in a stratified combustion mode, with fuel directly injected toward the piston bowl cavity during the compression stroke. In contrast to the above, at higher loads, the engine operates in a homogeneous combustion mode that a homogeneous mixture is introduced, with fuel injected during the intake stroke. During the intake stroke that homogeneous-charge early fuel injection is made, an in-cylinder pressure level is comparatively low, and thus the spray angle tends to increase or widen. The increased spray angle contributes to superior fuel atomization and creation of a homogeneous mixture suited to high load conditions. On the other hand, during the compression stroke that stratified-charge late fuel injection is made, the in-cylinder pressure level is comparatively high, and thus the spray angle tends to decrease or narrow. By virtue of the narrowed spray angle, the fuel spray can be effectively concentrated in the piston bowl cavity during the stratified combustion mode.
However, the previously-noted peculiar fuel spray characteristic of the swirl DI fuel injector is unsuitable to a DI gasoline engine that at least an installation position of a fuel injector and a spray angle are properly set, so that fuel spray injected on the compression stroke is directed directly to the vicinity of a tip of the spark plug and only a discharging gap of the spark plug is exposed to an upper part of the fuel spray, to create a richer air/fuel mixture layer around the spark plug. In case of the DI engine of the spray-angle setting and injector installation position suited to the stratified charge combustion mode, when the DI engine operates in the homogeneous combustion mode with fuel directly injected into the combustion chamber during the intake stroke, due to the relatively low in-cylinder pressure the spray angle tends to become wider than that of the stratified combustion mode. Owing to the undesirably widened spray angle, the fuel spray tends to be delivered to the whole body of the spark plug. This leads to the problem of reduced exhaust-emission control performance, increased unburned hydrocarbons (HCs), and increased deposits of sooty carbon.
In view of the above, it is desirable to achieve a high level of reconciliation of the homogeneous combustion mode and the stratified combustion mode by optimizing the shape of a tip of a swirl DI fuel injector. The previously described JP8-42427 proposes various shapes of swirl-fuel-injector orifice nozzle tips. JP8-42427 merely teaches the use of a so-called taper-cut orifice nozzle tip that creates a fuel spray of an inclined hollow cone whose axis is inclined with respect to the orifice axis. However, the swirl fuel injector disclosed in JP8-42427 is not designed to reconcile both the homogeneous charge combustion mode and the stratified charge combustion mode. In other words, JP8-42427 fails to teach the interrelationship between the shape of the fuel injector nozzle tip and the fuel spray characteristic that varies owing to an ambient pressure change during execution of fuel injection.
Accordingly, it is an object of the invention to provide an in-cylinder direct injection spark-ignition internal combustion engine capable of switching between a homogeneous charge combustion mode and a stratified charge combustion mode, in which a high level of reconciliation of the homogeneous charge combustion mode and the stratified charge combustion mode can be achieved by optimizing the shape of a tip of a swirl DI fuel injector.
In order to accomplish the aforementioned and other objects of the present invention, an in-cylinder direct injection spark-ignition internal combustion engine capable of switching between a homogeneous combustion mode that homogeneous-charge early fuel injection is executed on an intake stroke and a stratified combustion mode that stratified-charge late fuel injection is executed on a compression stroke, comprises a fuel injector having an orifice nozzle tip in which a nozzle hole is formed to inject fuel through the nozzle hole directly into a combustion chamber, a spark plug that provides a discharging gap in the combustion chamber, a swirl generating device that gives rotational momentum to the fuel flown into the nozzle hole, the orifice nozzle tip having an inclined surface that is formed on an end face of the orifice nozzle tip and inclined by a predetermined inclination angle with respect to a reference plane normal to an orifice axis of the nozzle hole, and the nozzle hole being formed within the inclined surface.
According to another aspect of the invention, an in-cylinder direct injection spark-ignition internal combustion engine capable of switching between a homogeneous combustion mode that homogeneous-charge early fuel injection is executed on an intake stroke and a stratified combustion mode that stratified-charge late fuel injection is executed on a compression stroke, comprises a fuel injector having an orifice nozzle tip in which a nozzle hole is formed to inject fuel through the nozzle hole directly into a combustion chamber, a spark plug that provides a discharging gap in the combustion chamber, a swirl generating device that gives rotational momentum to the fuel flown into the nozzle hole, the orifice nozzle tip having an inclined surface that is formed on an end face of the orifice nozzle tip and inclined by a predetermined inclination angle with respect to a reference plane normal to an orifice axis of the nozzle hole, the predetermined inclination angle being based on both an in-cylinder pressure dependent spray characteristic during the stratified-charge late fuel injection on the compression stroke and an in-cylinder pressure dependent spray characteristic during the homogeneous-charge early fuel injection on the intake stroke, and the nozzle hole being formed within the inclined surface.
According to a further aspect of the invention, an in-cylinder direct injection spark-ignition internal combustion engine capable of switching between a homogeneous combustion mode that homogeneous-charge early fuel injection is executed on an intake stroke and a stratified combustion mode that stratified-charge late fuel injection is executed on a compression stroke, comprises a fuel injector having an orifice nozzle tip in which a nozzle hole is formed to inject fuel through the nozzle hole directly into a combustion chamber, a spark plug that provides a discharging gap in the combustion chamber, a swirl flow component generating means for giving rotational momentum to the fuel flown into the nozzle hole, the orifice nozzle tip having an inclined surface formed on an end face of the orifice nozzle tip and inclined by a predetermined inclination angle with respect to a reference plane normal to an orifice axis of the nozzle hole for creating an inclined substantially hollow cone fuel spray, the predetermined inclination angle of the inclined surface being based on both an in-cylinder pressure dependent spray characteristic during the stratified-charge late fuel injection on the compression stroke and an in-cylinder pressure dependent spray characteristic during the homogeneous-charge early fuel injection on the intake stroke, and the nozzle hole being formed within the inclined surface.